Method of making silicon material with enhanced surface mobility by hydrogen ion implantation

ABSTRACT

A new-type silicon material is produced by hydrogen ion implantation and subsequent annealing, the annealing being preferably in two steps. The present invention raises surface mobility of a silicon wafer and produces a buried high-resistivity layer beneath a silicon surface layer. The resulting products are particularly useful for the improvement of yield and speed and radiation hardness of very large scale integrated circuits.

BACKGROUND OF THE INVENTION

The present invention relates generally to semiconductor processing and,more particularly, to processes of forming buried high-resistivitylayers and high-quality surface single crystal silicon layers with highelectron mobility.

Electron mobility in materials, substrate resistivity, and device sizeare three factors which affect speed (the speed of integrated circuits).Raising device speed can be achieved by increasing electron mobility inmaterials, increasing substrate resistivity, and shrinking the devicesize. For a long period of time, people have been looking forward tofinding a new material with high electron mobility, high resistivitysubstrate, and excellent crystal microstructure in order to manufactureultra high-speed, very large scale integrated circuits (VLSI).

Czochralski (CZ) silicon is relatively inexpensive and is the bestavailable material with acceptable microstructure. CZ silicon is usedworldwide for VLSI manufacturing at present. But mobility in CZ siliconcannot reach its theoretical limit value (i.e., intrinsic mobility)because of the existence of oxygen (about 1×10¹⁸ cm⁻³) and oxygenrelated defects. Thus, all attempts previously have failed to raise theresistivity of CZ silicon to higher than 100 Ωcm.

Silicon-on-insulator (SOI) technology is reaching the point of actualapplication to a manufacturable high-speed integrated circuit (IC) inrecent years. One of the more successful methods of SOI production isthe formation of a buried insulating layer by implantation of oxygen ornitrogen. But, substrates formed by oxygen implantation have thefollowing disadvantages: The damage in the surface region due to oxygenion bombardment is comparatively serious because the volume of oxygenion is comparatively large. In other words, oxygen implantation with adose of above 1×10¹⁸ cm⁻², that is too high, leads to many defects atthe surface region and the surface layer contains some defect-relatedoxygen precipitates produced during subsequent annealing. And thesedefects and oxygen precipitates seriously affect the shrinkage of thedevice size. In addition, the surface mobility in silicon wafersdecreases substantially after oxygen implantation and subsequentannealing.

Furthermore, other approaches in the prior art which have attempted toimprove SOI substrate by implantation such as, for example nitrogenimplantation cause problems similar to oxygen implantation. And evenSilicon-on-Sapphire (SOS) substrates in which Si films are grownheteroepitaxially on insulating sapphire substrates (i.e., SOI withoutimplantation), surface silicon layers contain the many defects becauseof lattice mismatch between Si and Al₂ O₃.

Another kind of important material used to manufacture semiconductordevices is GaAs. The two most favorable properties of GaAs are highelectron mobility and semi-insulating substrate which result in highspeed of GaAs ICs. Unfortunately, up to now control of crystalmicrostructure of GaAs material is still difficult and costly.

Many materials for use in manufacturing high-speed ICs have beeninvestigated in the past years. They can be grouped into three maincategories: GaAs, CZ silicon, and SOI substrate (including SOSsubstrate), all of which have serious disadvantages. For GaAs, presenttechnology for controlling crystal microstructure manufacture isexpensive. For CZ silicon, electron mobility and resistivity of thesubstrate are low. For SOI substrate, numerous defects at the surfaceregion results in low electron mobility and poor crystal microstructure.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to make a silicon waferhaving a single crystal silicon layer with higher electron mobilitycompared with original mobility.

It is a further object of the invention to form a high-resistivityburied layer beneath the surface silicon layer.

The invention can be summarized as a method of making a silicon waferhaving an improved single crystal silicon layer with improved electronicproperties on a defect layer produced by hydrogen ion implantation andsubsequent annealing. After hydrogen implantation, the silicon wafer issubjected to a first annealing step to form an interior layer ofhydrogen bubbles beneath the surface layer. During another annealingstep, the bubble-related defects getter the impurities in the surfacelayer. As a result, an improved single crystal silicon layer, withhigher electron mobility, on a high-resistivity defect-layer is formed.The surface region of a silicon wafer contains fewer impurities afterthe second annealing step than before implantation. Preferably, rapidthermal annealing (RTA) is utilized during the first annealing step asthe means to produce improved single crystal silicon surface layers withthe highest electron mobility. The present process overcomes thedisadvantages of prior art materials such as GaAs, CZ silicon, and SOImaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a new-type silicon materialaccording to the invention;

FIG. 2 depicts a depth profile of resistivity in the new-type siliconmaterial for the first embodiment of annealing scheme;

FIG. 3 depicts a depth profile of resistivity in the new-type siliconmaterial for the second embodiment of annealing scheme; and

Profiles (a) and (b) correspond to wafers in the orientation of (100)and (111), respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 2, the silicon wafer is implanted with hydrogenions at 180 keV to a dose of about 2.9×10¹⁶ H⁺ /cm², and the subsequentannealing conditions are RTA at 900° C. for 10 secs and thenconventional furnace annealing at 1180° C. for 20 min. The surface Hallelectron mobility in the sample is about 1400 cm² /Vs beforeimplantation and about 1760 cm² /Vs annealing at 1180° C. for 20 min.The surface Hall mobility increases by about 25% as compared with theoriginal mobility. This embodiment applies to a wafer in anyorientation.

The method of the invention utilizes hydrogen ion implantation andsubsequent annealing to form an improved single crystal silicon surfacelayer on a high-resistivity defect-layer in a silicon wafer. Siliconwafers are implanted with hydrogen ions, for example, at about 180 keVto a dose of about 2.7×10¹⁶ cm⁻², although both higher and lowerenergies and doses may be used. Subsequent annealing conditions aregrouped into two embodiments. The first embodiment is as follows: RapidThermal Annealing (RTA) and then conventional furnace annealing at hightemperature. The second embodiment is as follows: conventional furnaceannealing at comparatively low temperature and then conventional furnaceannealing at high temperature. In the second embodiment, however, thetime of first annealing step for the silicon in (100) orientation atcomparatively low temperature is much faster than in (111) orientation.The regrowth rates of implanted amorphous layers for differentorientation at comparatively low temperature have been reported by L.Csepregi et al in Appl. Phys. Lett., Vol. 29, p. 92, 1976. During firstannealing steps in the two embodiments, damage regions produced byhydrogen ion implantation epitaxially grow on the underlying undamagedsingle crystal silicon, and many interior hydrogen bubbles are formedbeneath surface layer after epitaxy of damage regions. These bubblescause structural defects around the bubbles. After the last annealingsteps, bubble-related defect layers are buried beneath surface layers.These interior bubbles do not move during annealing at high temperaturedue to their big volume. So the bubble-related defects have very highthermal stability of structure. And, the bubble-related defect layershave very high structural stability and a high resistivity of up to 10³Ωcm or even higher. In addition, the defect layers getter the impuritiesin the surface layers during last annealing steps at high-temperature,and impurity-related defects in surface layers disappear aftergettering. So defect-free (denuded) zones are formed at the surfacelayers. The removal of the impurities and defects from the surface layerresults in an increase of surface Hall mobility. As a result, entirelynew-type silicon materials are formed by hydrogen ion implantation andsubsequent annealing.

Referring now to FIG. 3, the silicon wafers are implanted with hydrogenions at 180 keV to the doses of about 2.7×10¹⁶ H+/cm². For a siliconwafer in (100) orientation, the subsequent annealing conditions areconventional furnace annealing at 600° C. for 20 min and thenconventional furnace annealing at 1180° C. for 20 min. For a siliconwafer in (111) orientation, the subsequent annealing conditions areconventional furnace annealing at 600° C. for 10 h and then conventionalfurnace annealing at 1180° C. for 20 min. Before implantation, thesurface Hall electron mobility in the wafers in (100) and (111)orientation is about 1400 cm² /Vs. After annealing at 1180° C. for 20min, the surface Hall electron mobility is about 1603 cm² /Vs for thewafer in (100) orientation and about 1542 cm² /Vs for the wafer in (111)orientation. The surface Hall electron mobility increases by about 15%for the silicon wafer in (100) orientation and by about 10% for thesilicon wafer in (111) orientation.

Referring now to FIGS. 1, 2 and 3, the increase of surface Hall electronmobility for the two embodiments shows that surface layer 1 containsmuch less impurities and defects than before implantation. In addition,the resistivity profile peaks 5, 8 and 11 correspond to the defect-layer2. FIGS. 2 and 3 show the resistivity of the buried defect layer 2 is upto 10³ Ωcm or even higher.

Significantly, I have discovered that other energy sources such asphoton energy from a laser may be used for the annealing steps. Moreadvantageously, the wavelength of the laser, such as a well-known CO₂laser or tunable NdYAG laser, can be selected for maximum photonabsorption in the irradiated region while minimizing the photonabsorption in the unirradiated zone. Also, the time of application ofthe energy source may be varied within the teachings of the invention toobtain optimum results. For example, the conventional furnace annealingat 1180° C. for 20 mins, described above in connection with FIG. 2, maybe advantageously increased to one hour to produce a single crystalsilicon layer and a stable defect layer. Tests by x-ray analysis on theSi on defect layer annealed for one hour indicated more perfect singlecrystal patterns. than that annealed for 20 mins. Furthermore, whendevices were fabricated on the Si on defect layer annealed for one hour,the defect layer was found to be stable after undergoing typical VLSImanufacturing process conditions. Thus, annealing time at 1180° C. ispreferably one hour or longer in the event a conventional furnace isused, but other times may be selected within the teaching of theinvention for other energy sources and other temperatures to providesingle crystal silicon on defect layer and to provide stability duringsubsequent cycling required for manufacturing processing.

Finally, I have discovered that under certain process conditions in VLSIproduction, my silicon on defect layer may cause wafer warpage due tobending stresses. To overcome this problem, I discovered that by forminga second defect layer by proton implantation through the second surfaceof the Si wafer using, for example, a dose of protons having similarenergy as that described above in connection with FIG. 3 to produce thefirst defect layer through the front surface region of the Si wafer, andafter annealing, I found that the bending stresses caused by the twodefect layers cancel each other, thereby causing the wafer to remainplanar without warpage which would otherwise interfere with subsequentVLSI manufacturing processing.

I claim:
 1. A method of forming a buried high-resistivity layercomprising hydrogen bubbles in a region beneath a surface layer of asilicon wafer having a (100) crystal orientation comprising the stepsof:implanting a dose of hydrogen ions into said silicon wafer; andheating said silicon wafer implanted with hydrogen ions, at a firsttemperature to form said buried high-resistivity layer comprisinghydrogen bubbles.
 2. The method recited in claim 1 wherein said hydrogenions are implanted at a dose of about 2.8×10¹⁶ H⁺ /cm² with an energy ofabout 180 keV.
 3. The method recited in claim 1 wherein said step ofheating includes conventional furnace annealing at about 600° C. forabout 20 minutes.
 4. The method recited in claim 1 further comprisingthe step of annealing said surface layer at a second temperature to formsingle crystal silicon.
 5. The method recited in claim 1 wherein saidstep of heating at said first temperature is carried out for a period oftime less than 8 hours.
 6. The method recited in claim 4 furthercomprising the step of fabricating semiconductor devices on said siliconwafer.